Background: The purine salvage enzyme inosine 5'-monophosphate (IMP)-specific 5'-nucleotidase catalyzes degradation of IMP to inosine. Although this enzymatic activity has been purified and characterized in Saccharomyces cerevisiae, the gene encoding IMP 5'-nucleotidase had not been identified.
Results: Mass spectrometry analysis of several peptides of this enzyme purified from yeast allowed identification of the corresponding gene as YOR155c, an open reading frame of unknown function, renamed ISN1. The deduced Isn1p sequence was clearly not homologous to 5'-nucleotidases from other species. However, significant similarities to Isn1p were found in proteins of unknown function from Neurospora crassa, Plasmodium falciparum and several yeast species. Knock-out of ISN1 resulted in the total loss of IMP-specific 5'-nucleotidase activity, thus confirming that the ISN1 gene indeed encodes the enzymatic activity purified from yeast. In vivo studies revealed that, when IMP is overproduced through constitutive activation of the IMP de novo synthesis pathway, ISN1 is required for excretion of inosine and hypoxanthine in the medium.
Conclusion: We have identified a new yeast gene, ISN1 (YOR155c), as encoding IMP-specific 5'-nucleotidase activity. The ISN1 gene defines a new type of 5'-nucleotidase which was demonstrated to be functional in vivo.